Additives for hydrogen/bromine cells

ABSTRACT

The invention relates to the use of 1-alkyl-2-alkyl pyridinium halide (e.g., 1-ethyl-2-methyl pyridinium bromide), 1-alkyl-3-alkyl pyridinium halide (e.g., 1-ethyl-3-methyl pyridinium bromide) or 1-alkyl-3-alkyl imidazolium halide (e.g., 1-butyl 3-methyl imidazolium bromide) as additives in an electrolyte used in hydrogen/bromine cells, for complexing the elemental bromine formed in such cells. The invention also provides an electrolyte comprising aqueous hydrogen bromide and said additives, and processes for operating an electrochemical flow cell selected from the group consisting of hydrogen/bromine or vanadium/bromine cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/210,976 filed Mar. 14, 2014, which claims the benefit of U.S.Provisional Application No. 61/781,141 filed on Mar. 14, 2013, and is acontinuation-in-part (CIP) of International Application No.PCT/IL2012/000349 filed on Sep. 23, 2012, which claims the benefit ofU.S. Provisional Application No. 61/537,622 filed Sep. 22, 2011, theentire contents of all of which are incorporated herein by reference.

The invention relates to compounds suitable as additives for anelectrolyte used in hydrogen/bromine cells, for complexing the elementalbromine formed in such cells.

There exists a need, in electrochemical flow cells which involve thegeneration of elemental bromine, to keep the bromine in a form which canbe readily stored and pumped over a broad temperature range, such thatit can be used without interfering with the operation of the flow cell.

The hydrogen/bromine cell is an example of a regenerative fuel cell. Theoperation of hydrogen/bromine regenerative fuel cells is based on theelectrolysis of hydrogen bromide, and the conversion of the electrolysisproducts, i.e., hydrogen and elemental bromine, back to hydrogenbromide. During charge, an electric current supplied from an externalsource drives the electrolysis of hydrogen bromide, generating hydrogen(H₂) and elemental bromine (Br₂), which are stored separately insuitable tanks located externally to the cell. H₂ and Br₂ are fed backto the cell during discharge and are reacted to give hydrogen bromide,thereby producing electric energy.

A characteristic hydrogen/bromine cell is shown diagrammatically inFIG. 1. While this figure depicts a single cell, it is to be noted thata plurality of such cells can be assembled in series. Numerals 1 and 2represent the hydrogen and bromine electrodes, respectively, and numeral3 represents a separator (e.g. an ion exchange membrane) positionedbetween the electrodes. The term “hydrogen electrode” is used herein toindicate the electrode where hydrogen gas is formed (during charge) andoxidized (during discharge). The term “bromine electrode” is used hereinto indicate the electrode where elemental bromine is formed (duringcharge) and reduced (during discharge).

A first storage tank, for collecting the hydrogen gas, is indicated bynumeral 4. A second storage tank, which contains a concentrated aqueoussolution of hydrogen bromide, is indicated by numeral 5. Flow paths 4 aand 5 c, connecting the hydrogen storage tank 4 and the HBr storage tank5 to the respective sides of the cell, and pumps for driving the fluidsalong the flow paths are also shown in FIG. 1.

The charge/discharge cycle is represented by the following pair ofchemical equations:

During charge, the electrolyte which comprises hydrobromic acid is fedfrom storage tank 5 to that side of the cell where the bromine electrode2 is placed (which is the cathodic side of the cell at the chargestate). The hydrobromic acid undergoes electrolysis, resulting in theformation of elemental bromine at the cathode. The electrolyte, enrichedwith elemental bromine, is removed from the cathodic side of the celland is transferred to the storage tank 5. Hydrogen ions concurrentlypass across the membrane 3 to the anodic side, where hydrogen gasevolves at the anode 1, and is collected in tank 4.

During discharge, the hydrogen gas and the bromine-containingelectrolyte are fed from their storage tanks 4 and 5, respectively, tothe respective sides of the cell, where the hydrogen and bromineelectrodes are positioned (it is to be noted that the anodic/cathodicsides are reversed relative to previous stage). The reaction betweenhydrogen and bromine yields hydrobromic acid, with electric currentbeing drawn from the cell.

It should be understood that the electrolyte, with which theelectrolysis stage starts, is not necessarily free of elemental bromine.In practice, the electrolysis stage starts with an electrolyte whichcontains, in addition to hydrobromic acid, also up to 10% elementalbromine. For example, the concentrations of HBr and Br₂ in the aqueouselectrolyte prior to the electrolysis may be from 5% to 52% (morepreferably 10% to 45%) and from 0% to 10% (more preferably 0.1% to 5%),respectively. During the electrolysis stage (i.e., the chargingprocess), the concentration of the hydrobromic acid in the electrolyteis gradually decreased, while the concentration of the elemental bromineincreases. Upon completion of the charge state, the electrolytetypically comprises from 5% to 35% HBr and from 0.2M to 3.5M Br₂. Itfollows that the composition of the electrolyte varies significantlyduring the charge/discharge cycle.

Bromine is a dark red, fuming liquid. It is reactive and corrosive andhas a high vapor pressure at room temperature. In cells utilizingbromine as an electrochemically active element, there is a need todeactivate the bromine, i.e., convert it into a form with reduced vaporpressure, which form is less likely to interfere with the operation ofthe cell. It is known in the art that this goal can be achieved byadding a bromine-complexing agent to the electrolyte. Thebromine-complexing agent combines with bromine molecule(s) to form apolybromide complex. As a result, the vapor pressure above the complexedbromine solution is decreased.

In regenerative fuel cells the electrolyte reservoir is separated fromthe electrodes stack, with the electrolyte being pumped from thereservoir to the electrodes and back. The flowability of the electrolytemust be maintained with respect to different compositions correspondingto different states of charge, and over the entire operationaltemperature range (typically between −15° C. and 50° C.). In otherwords, throughout the operation of the cell, the formation of a solidphase in the electrolyte is unacceptable.

In the past, bromine complexing agents were investigated for thedeactivation of elemental bromine in zinc bromine flow batteries.Bromine deactivation in these batteries may be achieved by the use ofcyclic quaternary ammonium bromides (abbreviated quats) as complexingagents. In their most general form, these salts are represented by thefollowing formula:

where R₁ and R₂ indicate the alkyl groups (which are generally differentfrom one another) and X indicates the halide counter ion. It should beparticularly noted that in this formula, the cation is a non-aromaticheterocyclic system. Specifically, N-methyl-N-ethyl pyrrolidiniumbromide (abbreviated MEP) and N-methyl-N-ethyl morpholinium bromide(abbreviated MEM) are both commercially used for that purpose. However,the experimental results reported below indicate that neitherN-methyl-N-ethyl pyrrolidinium bromide nor N-methyl-N-ethyl morpholiniumbromide is suitable for use in hydrogen/bromine regenerative fuel cells,for the reason that they crystallize under certain working conditionsemployed in such cells.

It has now been found that 1-alkyl-2-alkyl pyridinium bromide, such as1-ethyl-2-methyl pyridinium bromide (abbreviated 2-MEPy),1-alkyl-3-alkyl pyridinium bromide, such as 1-ethyl-3-methyl pyridiniumbromide (abbreviated 3-MEPy), and 1,3 dialkyl imidazolium bromide suchas 1-butyl 3-methyl imidazolium bromide (abbreviated BMIBr) and mixturesthereof are effective as complexing agents for a hydrogen/bromine cells,e.g., a hydrogen/bromine regenerative fuel cell. Having testedHBr/bromine containing electrolytes with varied compositionscorresponding to distinct states ensuing during the charge/dischargecycle of hydrogen/bromine cell, it has been surprisingly found that thepresence of said compounds in the electrolyte allows the formation ofpolybromide complexes which do not solidify under the relevant workingconditions.

Heretofore, 1-alkyl-2-methyl-pyridinium (also named N-alkyl picolinium)halide salts were proposed in the art for the following uses. U.S. Pat.No. 5,260,148, for example, describes the preparation of an electrolyticsolution for lithium secondary batteries by adding N-methyl picoliniumions to a solvent which is an equimolar mixture of propylene carbonateand 1,2-dimethoxyethane. Barlet, R. et al. [Journal de Chimie Physiqueet de Physico-Chimie Biologique (1984), 81(5), 349-54] describes the useof pyridinium halides as room temperature battery electrolytes. EP0404188 discloses a non-aqueous electrolytic aluminum plating bathcomposition comprising, inter alia, halide such as an N-alkyl picoliniumhalide. Shlyapnikov, D. S. [Khimiya Geterotsiklicheskikh Soedinenii(1972), (7), 966-9] describes SO₂ complexes with quaternary halide saltsof e.g. α-picoline.

The present invention is therefore primarily directed to the use of1-alkyl-2-alkyl pyridinium halide, 1-alkyl-3-alkyl pyridinium halide,1,3 dialkyl imidazolium halide or their mixtures, wherein the halide ispreferably bromide, as bromine complexing agents in electrochemical flowcells selected from the group consisting of hydrogen/bromine cell andvanadium/bromine cell. The alkyl groups attached to the aromatic ringare independently selected from the group of C1-C5 alkyl. Preferably,the alkyl groups are different from one another. In the case of1-alkyl-2-alkyl pyridinium bromide and 1-alkyl-3-alkyl pyridiniumbromide, it is preferred to have an ethyl group attached to the nitrogenatom and a methyl group attached to the carbon ring (i.e., either at the2- or 3-position of the pyridine ring).

In another aspect, the invention provides an electrolyte suitable foruse in electrochemical flow cells selected from the group consisting ofhydrogen/bromine cell and vanadium bromine cell, said electrolytecomprising aqueous hydrogen bromide and a liquid complex composed of atleast one of 1-alkyl-2-alkyl pyridinium halide (e.g., bromide),1-alkyl-3-alkyl pyridinium halide (e.g., bromide) or 1,3 dialkylimidazolium halide (e.g., bromide) combined with one or more brominemolecules. The liquid complex is preferably composed of 1-ethyl-2-methylpyridinium bromide, 1-ethyl-3-methyl pyridinium bromide or 1-butyl3-methyl imidazolium bromide and bromine molecules.

In another aspect, the invention is directed to a process for operatingan electrochemical flow cell selected from the group consisting ofhydrogen/bromine and vanadium bromine cell, comprising adding1-alkyl-2-alkyl pyridinium halide (e.g., bromide), 1-alkyl-3-alkylpyridinium halide (e.g., bromide), 1,3 dialkyl imidazolium halide (e.g.,bromide) or their mixtures as set forth above, to the HBr-containingelectrolyte of said cell.

The preferred complexing agents according to the invention,1-ethyl-2-methyl pyridinium bromide and 1-ethyl-3-methyl pyridiniumbromide are prepared by reacting 2-picoline or 3-picoline, respectively,with ethyl bromide, as illustrated by the following reaction schemes:

The reaction is carried out by charging a pressure reactor with thereactants and optionally also with a solvent, which may be either anaqueous or organic solvent. Alternatively, the reaction is solvent free,with one of the reactants being optionally used in excess. It ispossible to introduce the entire amounts of the reactants into thereactor and then start the reaction by heating the reaction mixture.However, it is also possible to gradually feed one or more of thereactants (e.g., the ethyl bromide) into the reactor over a period ofnot less than one hour under heating.

The reaction mixture is heated, preferably to a temperature of not lessthan 90° C., and the reaction is allowed to proceed under pressure for afew hours. The product is conveniently collected in the form of anaqueous solution, which can be directly applied as an additive for theHBr electrolyte solution in accordance with the present invention. Tothis end, upon completion of the reaction, the organic solvent and/orresidual amounts of the starting materials are removed from the reactionvessel by means of methods known in the art, e.g., distillation. Watercan then be added into the reactor, to afford the complexing agent in anaqueous form. The concentration of the aqueous solution of 2-MEPy or3-MEPY or their mixture which can be used as an additive for the HBrelectrolyte is preferably from 50 to 90 wt %.

Another complexing agent suitable for use according to the invention,1-butyl 3-methyl imidazolium bromide, is commercially available fromChemada Israel, and can be also prepared by methods known in the art.

The electrolyte according to the invention is prepared by combiningtogether aqueous hydrogen bromide, the complexing agent, e.g.,1-ethyl-2-methyl pyridinium bromide, 1-ethyl-3-methyl pyridiniumbromide, 1-butyl 3-methyl imidazolium bromide or a mixture thereof, andthe electrochemically generated bromine, which is formed in-situ in thecell on charging, or chemically (e.g., peroxide) generated bromine. Toan aqueous solution of hydrogen bromide, with HBr concentration of5%-52% by weight, e.g., 10-wt %, will be added the complexing agent suchthat its concentration in the resulting solution is not less than 0.25M,up to a concentration capable of complexing the maximal bromine content.On charging, the hydrogen bromide is consumed and bromine is generated.On discharging, the aqueous phase of the electrolyte is againconcentrated with respect to HBr, and the concentration of bromine isreduced.

As noted above, the bromine complexing agents of the invention may beused either in individual form or in the form of mixtures, e.g., binarymixtures, in which the molar ratio between the two components of themixture may be from 1:5 to 5:1, more preferably from 1:4 to 4:1 and evenmore preferably, from 1:3 to 3:1. One preferred mixture consists of1-ethyl-2-methyl pyridinium bromide and 1-ethyl-3-methyl pyridiniumbromide in a molar ratio between 1:2 to 1:4. The comlexing agents arepreferably added to the electrolyte in the form of concentrated aqueoussolutions in which the concentration of the complexing agent may be from40 to 92% by weight, e.g., 65 to 90% by weight.

The process of the invention is carried out utilizing a hydrogen/brominecell of the type described above with reference to FIG. 1, with theaddition of the complexing agent into the storage tank used for holdingthe aqueous HBr (indicated by numeral 5 in FIG. 1). Another example ofhydrogen/bromine cell which can be used in the process of the inventionis illustrated in U.S. Pat. No. 4,520,081. Vanadium/bromine cells whichcan be operated according to the invention are illustrated, for example,in U.S. Pat. No. 7,320,844 or US 2006/0183016.

The invention also relates to a structural material suitable for use inthe construction of hydrogen/bromine energy storage device. Suchstructural materials ought to exhibit a combination of high mechanicalstrength and good chemical resistance over broad range of workingtemperatures, due to the highly corrosive nature of the HBr/Br₂ aqueouselectrolyte circulating in the cell. For example, in the usualconstruction of hydrogen/bromine energy storage device, a plurality ofcells such as the one illustrated in FIG. 1 are assembled togetheradjacent to one another in a stack configuration to produce the desiredvoltage within the stack. Various parts of the hydrogen/bromine energystorage device, such as the tank used to hold the bromine-containingaqueous HBr electrolyte, and pipes used to supply and withdraw thereactants and reaction products to and from the cell stacks, arecontinually exposed to elemental bromine and hydrobromic acid, which areboth corrosive substances. Therefore, structural materials inhydrogen/bromine energy storage device must be chosen carefully.

The problem of finding a structural material suitable for use inhydrogen/bromine-based systems was addressed in U.S. Pat. No. 4,520,081,where various materials for making the frame of the cell which surroundsthe electrodes are considered, including inert plastics and specificallypolytetrafluoroethylene, i.e., fluorine-containing polymers which arewell known for their high chemical inertness. In U.S. Pat. No.4,520,081, a modified form of graphite having a layer of pyrographitedisposed on its surface was used for making the frames of the cells.

High density polyethylene (HDPE) is increasingly used as a structuralmaterial, for example in piping systems for gas distribution and waterlines. However, HDPE is not considered as a structural material ofchoice in systems where exposure to bromine is expected to occur.Indeed, the experimental work conducted in support of this inventionindicates that HDPE is incompatible with the electrolyte operable inhydrogen/bromine cells, as it is unable to withstand an attack byHBr/Br₂ aqueous solution under the relevant working conditions, e.g., ata temperature of 50° C.; under these conditions, HDPE is severelydamaged.

We have now found that HDPE can serve as a structural material in energystorage devices comprising hydrogen/bromine cells, if an additiveselected from the group consisting of 1-alkyl-2-alkyl pyridinium halide,1-alkyl-3-alkyl pyridinium halide or their mixture (e.g., 2-MEPy, 3-MEPyor a mixture thereof) is added to the electrolyte, i.e., to the aqueoushydrogen bromide solution. The experimental results reported belowindicate that when said additive(s) is(are) present in the electrolytesolution, then HDPE can withstand the electrolyte corrosiveness and iscapable of maintaining its mechanical properties.

Accordingly, the invention also relates to an energy storage devicecomprising:

-   -   a plurality of hydrogen/bromine cells arranged in a stack        configuration, each cell having therein spaced apart bromine and        hydrogen electrodes which are in electrical contact with means        for supplying electrical current to the cell and collecting        electrical current generated by the cell; a separator positioned        in the space between said electrodes dividing the cell into        first and second compartments; and an aqueous hydrogen bromide        electrolyte in which 1-alkyl-2-alkyl pyridinium halide (for        example, 1-ethyl-2-methyl pyridinium bromide), 1-alkyl-3-alkyl        pyridinium halide (for example, 1-ethyl-3-methyl pyridinium        bromide) or a mixture thereof is present;    -   hydrogen storage tank and HBr/Br₂ aqueous electrolyte storage        tank connected by means of conduits to the cell compartments;        wherein at least one component of said device (e.g., a tank        and/or a conduit used for electrolyte storage and circulation)        is made of HDPE.

Hydrogen/Bromine cell which can be used in the energy storage device ofthe invention contains hydrogen electrode (which may be made of carboncovered with platinum, supported on one face of the membrane); bromineelectrode (e.g., in the form of a carbon felt); cell frames made ofgraphite; sulfonated polytetrafluoroethylene (e.g., Nafion®) membrane;current collectors and end plates, as described, for example, in U.S.Pat. No. 4,520,081. An energy storage device based on hydrogen/brominecells is described in US 2012/0299384; parts of such a device, which maybe made of HDPE according to the present invention, include theelectrolyte storage tank and electrolyte feed lines. HDPE for use in theconstruction of the cell preferably has a density greater than 0.941g/cm³, e.g., greater than 0.945 g/cm³. For example, PE-HWST which iscommercially available from SIMONA can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a typical hydrogen/bromine cell.

FIG. 2 is photograph of a reference white test HDPE specimen and a testspecimen following exposure to an electrolyte solution which containsHBr and bromine.

FIG. 3 is photograph of a reference white test HDPE specimen and a testspecimen following exposure to an electrolyte solution which containsHBr, bromine and the additives of the invention.

The following non-limiting working examples illustrate various aspectsof the present invention.

EXAMPLES

Methods

-   -   1) The specific conductivity of the hydrogen bromide acid        solutions containing the complexing agents were measured at room        temperature, before the addition of bromine to the samples using        Innolab 740 instrument with graphite conductivity cell.

2) The temperature at which the formation of a solid phase takes placein the electrolyte solution was determined by gradually cooling thesamples from RT (approximately 25-30° C.) to −15° C. The cooling regimewas as follows: the temperature was decreased from RT down to 15° C.with a cooling rate of 0.2° C./min, and kept at 15° C. for 4 hours andso forth down to −15° C. At each of the following temperatures: 15° C.,10° C., 5° C., 0° C., −5° C., −10° C. and −15° C., the solution wasmaintained at a constant temperature for four hours. The cooling testwas performed in polyethylene glycol solution, until the formation ofcrystals was observed.

3) The bromine concentration in the aqueous phase above the polybromidecomplex-oily phase was determined by a conventional iodometric titrationtechnique. Each vial was sampled three times at room temperature.

4) The vapor pressure above the electrolyte solutions containing thecomplexing agents was measured at 20-26° C. according to “Vaporpressures of bromine-quaternary ammonium salt complexes for zinc-brominebattery applications” Satya N. Bajpal J. Chem. Eng. Data 1981, 26, 2-4.

Example 1 Preparation of 2-MEPy in Aqueous Medium

A pressure reactor was equipped with a mechanical stirrer with amagnetic relay and a thermocouple well. The reactor was purged withnitrogen, charged with 2-picoline (101.3 g) and de-ionized water (DIW)(20 mL), sealed and the mixture was heated to 92° C. Ethyl bromide (97.9g) was slowly added during 3 hours, at 92-100° C. The mixture was heatedat 94-100° C. for additional 2 hours, then cooled, and the pressure wasreleased. The crude solution was diluted with DIW (24 mL) and excess2-picoline was distilled-off as aqueous azeotrope, under reducedpressure. Finally, the residue was diluted with DIW. Final product: 251g; 66.1 weight % (argentometric titration); yield, 91.5%.

Example 2 Preparation of 2-MEPy in Acetonitrile as a Solvent

A pressure reactor was equipped with a mechanical stirrer with amagnetic relay and a thermocouple well. The reactor was purged withnitrogen, charged with 2-picoline (57.9 g), ethyl bromide (69 g) andacetonitrile (69 g). The reactor was sealed and the mixture heated to97° C. Heating at 97° C. was continued for 6 hours. Distillation of thesolvent was controlled by the upper valve of the reactor followed byvacuum distillation (without cooling). DIW (31 mL) was added to dissolvethe crude mixture and vacuum was applied to remove residualacetonitrile. Finally, the solution was diluted with DIW (10.5 g). Finalproduct: 149 g; 80.0 weight % (argentometric titration); yield, 95%.

Example 3 Preparation of 2-MEPy with Excess Ethyl Bromide

A pressure reactor was equipped with a mechanical stirrer with amagnetic relay and a thermocouple well. The reactor was purged withnitrogen, charged with 2-picoline (95 g) and ethyl bromide (145 g). Thereactor was sealed and the mixture heated to 97° C. Heating at 97° C.was continued for 18 hours. Distillation of excess ethyl bromide wascontrolled by the upper valve of the reactor followed by vacuumdistillation. Finally, the solution was diluted with DIW (47 g). Finalproduct: 250 g; 79.3 weight % (argentometric titration); yield, 96%.

Example 4 Preparation of 3-MEPy or 4-MEPy

A pressure reactor was equipped with a mechanical stirrer with amagnetic relay and a thermocouple well. The reactor was purged withnitrogen, charged with 3-picoline (101.3 g) and DIW (25 mL). The reactorwas sealed and the mixture was heated to 96° C. Ethyl bromide (97.9 g)was slowly added during 2 hours, at 96-104° C. The mixture was heated at100° C. for additional 3.5 hours, after which time the pressure wasreleased. The crude solution was diluted with DIW and excess 3-picolinewas distilled-off as aqueous azeotrope, under reduced pressure. Finally,the residue was diluted with DIW. Final product: 260 g; 66.6 weight %(argentometric titration); yield, 95.6%. 4-MEPy was prepared in asimilar manner, starting from 4-picoline.

Examples 5-7 (of the Invention) and 8-10 (Comparative) Preparing andMeasuring the Properties of Electrolyte Solutions which Correspond toElectrolyte Solutions at the Beginning of the Charge Stage

Samples of hydrogen bromide acid solutions containing the complexingagents (abbreviated Quats), were prepared with final HBr concentrationof 34% by weight, 0.8M of Quat and 0.2M of bromine, namely, with acomposition corresponding to the composition of an electrolyte at thebeginning of a charging process in hydrogen/bromine cell. The totalvolume of each sample was 12 ml in a closed vial. The samples werestored at room temperature (RT) for 24 hours after preparation beforeany measurement was conducted. The samples were tested for the followingproperties: the temperature at which a solid phase is formed in theelectrolyte, free bromine concentration, conductivity and vaporpressure. The results are given in the following table:

TABLE 1 Temperature at which a [Br₂] in Specific Vapor solid phaseaqueous conductivity pressure Ex. Quat was observed phase (%) (mS/cm)(mm Hg) 5 2-MEPy  5° C. 0.9  588 24 6 3-MEPy −5° C. 0.65 623 24 7 BMIBr−10° C.  0.13 534 22 8 4-MEPy 25° C. N/A 610 N/A 9 MEP 20° C. N/A 600N/A 10 MEM 20° C. N/A 597 N/A

The results show that 2-MEPy, 3-MEPy and BMIBr are suitable for use asbromine complexing agents in electrolyte solutions of hydrogen/brominecells at the beginning of the charge state.

Example 11-13 (of the Invention) and 14-16 (Comparative) Preparing andMeasuring the Properties of Electrolyte Solutions which Correspond toElectrolyte Solutions at the End of the Charge Stage

The procedures and measurements as set forth in the previous exampleswere repeated. However, the amounts of HBr and elemental bromine wereadjusted to form samples of electrolyte solutions that represent thecomposition of the electrolyte at the end of the charge process. Hence,samples were prepared with final HBr concentration of 22% by weight,0.8M of Quat and 1M of bromine. The total volume of each sample was 12ml in a closed vial. The samples were stored at room temperature for 24hours after preparation before any measurement was conducted. Thesamples were tested for the following properties: the temperature atwhich a solid phase is formed in the electrolyte, free bromineconcentration, conductivity and vapor pressure. The results are shown inTable 2.

TABLE 2 [Br₂] in Specific Vapor solidification aqueous conductivitypressure Ex Quat temperature phase (%) (mS/cm) (mm Hg) 11 2-MEPy −10°C.  1.05 582 21 12 3-MEPy −10° C.  0.99 605 18 13 BMIBr −7° C. 0.85 53019 14 4-MEPy 25° C. N/A 597 N/A 15 MEP  5° C. 1.21 593 24 16 MEM  5° C.2.14 591 30

The results show that 2-MEPy, 3-MEPy and BMIBr are suitable for use asbromine complexing agents in electrolyte solutions of hydrogen/brominecells at the end of the charge stage.

Examples 17-21 (of the Invention) and 22-23 (Comparative)

In this set of examples, various mixtures of complexing agents weretested in electrolyte solutions having a composition which correspondsto the composition of an electrolyte solution at the beginning of thecharge process (see the procedure of Examples 5-10). The results aretabulated in Table 3.

TABLE 3 [Br₂] in Specific aqueous conduc- Vapor Quats Quatssolidification phase tivity pressure Ex mixture ratio temperature (%)(mS/cm) (mmHg) 17 2-MEPy/3- 3:1  −5° C. 0.93 561 22 MEPy 18 2-MEPy/3-1:1 −10° C. 0.81 456 35 MEPy 19 2-MEPy/3- 1:3 −10° C. 0.49 600 36 MEPy20 2-MEPy/3- 1:1:1 −10° C. 1.38 598 — MEPy/4- MEPy 21 2-MEPy/4- 3:1  −5°C. 0.8 572 — MEPy 22 2-MEPy/4- 1:1  20° C. N/A N/A N/A MEPy 23 2-MEPy/4-1:3  20° C. N/A N/A N/A MEPy

Examples 24-29 (of the Invention) and 30 (Comparative)

In this set of examples, various mixtures of complexing agents weretested in electrolyte solutions having a composition which correspondsto the composition of an electrolyte solution at the end of the chargeprocess (see the procedure of Examples 11-16). The results are tabulatedin Table 4.

TABLE 4 [Br₂] in Specific aqueous conduc- Vapor Quats Quatssolidification phase tivity pressure Ex mixture ratio temperature (%)(mS/cm) (mmHg) 24 2-MEPy/3- 3:1 −10° C. 1.12 569 27 MEPy 25 2-MEPy/3-1:1 −10° C. 0.88 562 34 MEPy 26 2-MEPy/3- 1:3 −10° C. 1.16 566 38 MEPy27 2-MEPy/3- 1:1:1 −10° C. 1.12 576 — MEPy/4- MEPy 28 2-MEPy/4- 3:1 −10°C. 0.92 571 43 MEPy 29 2-MEPy/4- 1:1  −5° C. 1.25 — — MEPy 30 2-MEPy/4-1:3  20° C. — — — MEPy

Examples 31-36

Samples were prepared with HBr concentration of 10% by weight. Thecomplexing agent that was tested was 2-MEPy. The concentration of 2-MEPyin each sample was 0.8M. Different amounts of elemental bromine wereadded to the samples and some properties of interest (the concentrationof elemental bromine in the aqueous phase, the conductivity and vaporpressure) were measured at two temperatures: 22° C. and 45° C.

The following is noted with respect to the measurements relevant to thisset of examples (31-36) and the next two sets of examples (37-42 and43-48):

The samples were stored at 25° C. for at least 24 hours afterpreparation before any measurement was conducted.

Conductivity measurements were carried out at 22-24° C. on solutionswhich contain bromine.

Sample preparations for iodometric titration and the titration itselfwere done at 22-24° C.

The equilibrium total pressure above the electrolyte at the desiredtemperature has been measured using mercury manometer vs equilibriumpressure of liquid for which exact values of equilibrium vapor pressureare well known in all range of temperatures. Distillated water was usedas the reference. Two round-bottom flasks of the same volume and withthe same volume of the measured electrolyte and water, closed by vacuumvalves, were connected to the mercury manometer. Each flask wasaccurately equilibrated at the desired temperature and the vacuum valveswere opened. After the system was equilibrated the difference betweenthe levels of mercury in both side of manometer tube was measured. Theaccurate value of water pressure at temperature of water flask is known.The measured difference in mercury levels has been added to this value.

The results are tabulated in Table 5.

TABLE 5 [Br₂] in aqueous Vapor pressure Vapor pressure Br₂, M phaseConductivity, at 22° C., at 45° C., Ex. added (%) mS/cm mmHg mmHg 31 — —352 17 78 32 1.0 0.42 479 — — 33 1.5 0.79 480 — — 34 2 1.92 485 10 63 352.5 3.72 481 14 77 36 3 6.07 479 18 82

Examples 37-42

Samples were prepared with HBr concentration of 10% by weight. Thecomplexing agent that was tested was a mixture consisting of 2-MEPy and3-MEPy at molar ratio of 3:1. The concentration of the mixture of 2-MEPyand 3-MEPy in each sample was 0.8M. Different amounts of elementalbromine were added to the samples and some properties of interest (theconcentration of elemental bromine in the aqueous phase, theconductivity and the vapor pressure) were measured at threetemperatures: 22° C., 45° C. and 60° C. The results are tabulated inTable 6.

TABLE 6 Vapor [Br₂] [Br₂] pressure in aq. in aq. Vapor Vapor at Br₂,phase phase pressure pressure 60° C., M (%) (%) Cond. at 22° C., at 45°C., ±3 Ex. added 22° C. 45° C. mS/cm ±1 mmHg ±2 mmHg mmHg 37 — 365 15 80165 38 1.0 0.64 0.75 460 — — — 39 1.5 0.95 1.20 478 — — — 40 2 1.17 2.73475 — — — 41 2.5 2.79 5.31 467 16 75 176 42 3 5.16 8.76 462 17 77 185

Examples 43-48

Samples were prepared with HBr concentration of 10% by weight. Thecomplexing agent that was tested was a mixture consisting of 2-MEPy and3-MEPy at molar ratio of 1:3. The concentration of the mixture of 2-MEPyand 3-MEPy in each sample was 0.8M. Different amounts of elementalbromine were added to the samples and some properties of interest (theconcentration of elemental bromine in the aqueous phase, theconductivity and the vapor pressure) were measured at threetemperatures: 22° C., 45° C. and 60° C. The results are tabulated inTable 7.

TABLE 7 [Br₂] [Br₂] Vapor in aq. in aq. Vapor Vapor pressure Br₂, phasephase pressure pressure at M (%) (%) Cond. at 22° C., at 45° C., 60° C.,Ex. added 22° C. 45° C. mS/cm mmHg mmHg mmHg 43 — 380 17 74 158 44 1.00.69 1.90 463 — — — 45 1.5 0.94 1.79 472 — — — 46 2 1.40 2.15 475 — — —47 2.5 2.82 5.56 465 17 67 170 48 3 6.66 8.36 466 17 70 183

It is apparent from Tables 5, 6 and 7 that at temperatures of 22° C. and45° C., the increase at the amount of elemental bromine in theelectrolyte does not result in an increase of the vapor pressure,indicating that the additives of the invention form strong complexeswith the elemental bromine in HBr solutions. It should be noted that ata temperature of 60° C. a small increase of the vapor pressure isobserved, but this temperature is beyond the temperature range at whichelectrochemical cells normally operate.

Example 49 (Comparative) and Example 50 (of the Invention)

HDPE test specimens (PE-WHST™ with density of 0.947 g/cm³, availablefrom SIMONA) were exposed to aqueous HBr solutions which containelemental bromine under the experimental conditions set forth below andwere then subjected to various tests. The average dimensions of the testspecimens were as follows: length—6.2 cm; width—1.2 cm; thickness—0.3cm.

In a first set of experiments, a test specimen was immersed in 200 mlaqueous solution which contains hydrogen bromide and elemental bromineat concentrations of 10 wt % and 3M, respectively. The solution washeated to 50° C. under reflux. The solution was maintained understirring at 50° C. for a period of time of 30 days, following which thetest specimen was removed from the solution.

A second set of experiments was carried out similarly to the first one,with the difference that a mixture of 2-MEPy and 3-MEPy at aconcentration of 0.8M was present in the solution. The molar ratiobetween 2-MEPy and 3-MEPy in the mixture was 3:1.

The test specimen which was removed from the solution was inspectedvisually, to evaluate color and structural changes occurring on thesurface of the specimen. The test specimen was also weighed immediatelyat the end of the experiment. The mechanical stability of the testspecimens was assessed by comparing the (average) impact strength withthat of a reference specimen; the impact strength was measured using theIzod notched test (ASTM D-256-92 with pendulum of 10.8 J). Theexperimental conditions and results are tabulated in Table 8. Examples49 and 50 correspond to the first and second sets of experiments,respectively.

TABLE 8 Example 49 (comparative) 50 Experimental conditions Testspecimen HDPE HDPE Composition of the aqueous HBr 10 wt % HBr 10 wt %solution Br₂ 3M Br₂ 3M 2-MEPy + 3-MEPy Temperature 50° C. 50° C.Properties of the specimen Visible properties Color change from Colorchange from white-to- white to yellow- reddish brown; orange; surfaceBlisters were remained unchanged formed on the surface of the specimenweight change (%)  13.41^(a)  2.4^(c) Impact strength 679^(a) 638^(c)(Izod notched J/m) (Reference: 622^(b)) (Reference: 636d) ^(a)theaverage of 10 measurements ^(b)the average of 10 measurements ^(c)theaverage of 10 measurements ^(d)the average of 10 measurements

Regarding the results of Example 49, it is noted that HDPE testspecimens exposed to bromine-containing aqueous hydrogen bromidesolutions absorbed an appreciable amount of elemental bromine, asindicated by the severe color change and the large increase in theweight of the specimen. FIG. 2 is photograph of a reference white testspecimen and a typical test specimen obtained following the experiment(on the left and right sides of the photograph, respectively; thephotograph was taken before the Izod notched test). The photograph showsthe severe color change. In addition, the surface of the HDPE testspecimen was seriously damaged, as indicated by the formation of smallblisters marked by an arrow in FIG. 2, i.e., small “pockets” with liquidtrapped therein.

Regarding the results of Example 50, it is noted that in the presence ofthe complexing agents, the resistance of HDPE to the electrolyte ismarkedly improved. The amount of bromine absorbed by the HDPE testspecimen is acceptable, as indicated by a small increase in the weightof the specimen and the white-to-yellow/orange color change. FIG. 3 isphotograph of a reference, white test specimen and a typical testspecimen obtained following the experiment (on the lower and upper sidesof the photograph, respectively; the photograph was taken after the Izodnotched test). No damage is observed on the surface of the HDPE testspecimen. Furthermore, HDPE exposed to elemental bromine in the presenceof the complexing agents retains its mechanical strength as indicated bythe fact that the impact strength of the reference specimen and the testspecimen are comparable.

1) An electrolyte suitable for use in an electrochemical flow cell, saidelectrolyte comprising aqueous hydrogen bromide and a liquid complexcomposed of at least one of 1-alkyl-2-alkyl pyridinium halide,1-alkyl-3-alkyl pyridinium halide or 1-alkyl-3-alkyl imidazolium halidecombined with one or more bromine molecules. 2) An electrolyte accordingto claim 1, comprising a liquid complex composed of at least one of1-ethyl-2-methyl pyridinium bromide, 1-ethyl-3-methyl pyridinium bromideor 1-butyl 3-methyl imidazolium bromide combined with one or morebromine molecules. 3) An electrolyte according to claim 2, comprising aliquid complex composed of at least one of 1-ethyl-2-methyl pyridiniumbromide or 1-ethyl-3-methyl pyridinium bromide, combined with one ormore bromine molecules. 4) An electrolyte according to claim 3,comprising a mixture of 1-ethyl-2-methyl pyridinium bromide and1-ethyl-3-methyl pyridinium bromide. 5) Use of at least one of1-alkyl-2-alkyl pyridinium halide, 1-alkyl-3-alkyl pyridinium halide or1-alkyl-3-alkyl imidazolium halide as bromine-complexing agents in anelectrochemical flow cell which contains an electrolyte comprisingaqueous hydrogen bromide, said cell being hydrogen/bromine cell orvanadium bromine cell. 6) Use according to claim 5, wherein the1-alkyl-2-alkyl pyridinium halide is 1-ethyl-2-methyl pyridiniumbromide. 7) Use according to claim 5, wherein the 1-alkyl-3-alkylpyridinium halide is 1-ethyl-3-methyl pyridinium bromide. 8) Useaccording to claim 5, wherein a mixture of 1-ethyl-2-methyl pyridiniumbromide and 1-ethyl-3-methyl pyridinium bromide is used. 9) A processfor operating an electrochemical flow cell selected from the groupconsisting of hydrogen/bromine or vanadium/bromine cells, comprisingadding to HBr-containing electrolyte solution of said cell an additiveselected from the group consisting of 1-alkyl-2-alkyl pyridinium halide,1-alkyl-3-alkyl pyridinium halide, 1-alkyl-3-alkyl imidazolium halide ortheir mixture. 10) A process according to claim 9, comprising adding1-ethyl-2-methyl pyridinium bromide, 1-ethyl-3-methyl pyridinium bromideor a mixture thereof to the HBr-containing electrolyte inhydrogen/bromine cell. 11) An energy storage device comprising: aplurality of hydrogen/bromine cells arranged in a stack configuration,each cell having therein spaced apart bromine and hydrogen electrodeswhich are in electrical contact with means for supplying electricalcurrent to the cell and collecting electrical current generated by thecell; a separator positioned in the space between said electrodesdividing the cell into a first and second compartments; and an aqueoushydrogen bromide electrolyte in which 1-alkyl-2-alkyl pyridinium halide,1-alkyl-3-alkyl pyridinium halide or a mixture thereof is present;hydrogen storage tank and HBr/Br₂ aqueous electrolyte storage tankconnected by means of one or more conduits to the cell compartments;wherein at least one component of said device comprises high densitypolyethylene (HDPE). 12) An energy storage device according to claim 11,wherein the electrolyte storage tank and/or conduit(s) used forelectrolyte circulation comprise HDPE. 13) An energy storage deviceaccording to claim 11, wherein the 1-alkyl-2-alkyl pyridinium halide and1-alkyl-3-alkyl pyridinium halide are 1-ethyl-2-methyl pyridiniumbromide and 1-ethyl-3-methyl pyridinium bromide, respectively.